1. Field of Invention
This invention relates to ballistic armor. More specifically, this invention relates to a transparent armor system utilizing a multi-layer structure incorporating glass-ceramics and laminates.
2. Description of the Related Art
In order to provide protection of personnel and equipment from ballistic projectiles, explosive ordnance, and forces and objects from detonation of improvised explosive devices (collectively hereinafter “projectiles”), it is necessary to provide a means of disbursing the kinetic energy of such projectiles to prevent them from reaching their target. Although this may be accomplished by interposing a large mass of any of a number of different materials between the target and the incoming projectile, experience has shown that a much more efficient means of energy disbursement is provided by suitably engineered ballistic armor structures wherein layers of different materials act to disrupt and disperse the energy of an incoming projectile. Such structures strive to maximize the amount of material which may be acted upon to absorb and disburse the energy of the projectile, while at the same time breaking or deforming the projectile and distributing these resulting fragments into a wider area. Such structures further strive to minimize the total amount of materials required for the protection of a specific area.
Ballistic armor structures generally contain one or more layers of material engineered to spread the force of the impact by deforming, deflecting, or fragmenting the ballistic projectile while the ballistic armor itself undergoes deformation or localized fragmentation. The deformation and localized fragmentation processes of the ballistic armor structure absorb a large portion of energy from the projectile while simultaneously spreading the impacted area to involve more material in successive layers. Both hardness and toughness of the ballistic armor structure are required for these functions.
In the field of ballistic armor structures, the initial layer of material used to disrupt the incoming ballistic projectile is often referred to as the “strike face,” or alternatively, the “hard face.” The hard face is typically a layer of relatively hard and tough material designed to deform, and in some cases fragment, to absorb at least some of the energy of the incoming projectile, thereby distributing the projectile's energy. Following the hard face are other layers specifically designed to absorb the remaining energy of the impacting material and pieces of the previous hard face. These layers are often referred to as the “backing” or “catcher.”
The process of energy absorption and disbursement of the incoming projectile by the ballistic armor structure is generally intended to result in deformation, displacement and/or localized fracture of the hard face, and deformation and/or displacement of the backing, but without penetration through the ballistic armor structure by any fragments of the ballistic projectile. Selection of materials for these distinct functions and careful attention to construction and coupling of the various layers is essential to optimizing performance of the ballistic armor structure.
Great advances have been made in selection of materials for optimizing the performance of ballistic armor structures. Use of high-strength, hard, and in some cases “tough” ceramics like aluminum oxide, boron carbide, titanium diboride and silicon carbide for the hard face; and rigid or soft laminates of fibrous materials such as fiberglass, aramid, or polyethylene fiber for the backing have greatly reduced the mass and bulk of protective structures. These advances, unfortunately, have not been readily applicable to those areas where a transparent protective structure is required. Neither the high-strength, hard ceramics nor the laminated fibrous backing materials are typically transparent, and so neither are adaptable to transparent protective structures.
The need for transparency severely limits possible choices of materials for fabrication of the hard face of transparent protective systems. Although recent advances have been demonstrated in use of hot-pressed spinel or aluminum oxynitride (ALON) ceramics, or melt grown aluminum oxide (sapphire) crystal sheets for the hard face, manufacturing cost and size limitations would seem to restrict their use in all but the most critical of situations. The standard material used for fabrication of the hard face in transparent structures is borosilicate float glass or soda lime glass, a material which is neither very hard, nor very tough, and which has a relatively high specific density. This results in the need to greatly increase the aerial mass and bulk of transparent armors in order to preserve effectiveness. Such increase in aerial mass and bulk ultimately results in a conventional transparent armor having an increased weight per level of protection provided by the transparent armor.
A similar situation exists in regard to the materials used in the backing layers. The fibrous laminates traditionally used in the backing layers of ballistic armor structures are not transparent. Traditional backing and fragment catching layers for transparent armor are predominately un-reinforced sheets of polyacrylic or polycarbonate polymer, although some advances have been made in the use of optimized copolymer compositions for these layers. Thus, for most transparent armor applications, the chosen solution is the same as that which has been used for decades, a hard face of multiple layers of borosilicate float glass with a backup layer or layers of a polymer sheet to catch fragments, bound together with a conventional transparent adhesive.
Recent developments in the requirements and testing standards for transparent armor systems for use by the United States military, police, and other such organizations have introduced a need for transparent armor having an increased ability to protect against multiple subsequent impacts from multiple projectiles. For example, under the purchase specification ATPD 2352, published on Jul. 7, 2008 for use by the Department of the Army and the Department of Defense, transparent armor for use by the United States army must be capable of stopping a series of four shots of a specific rifle ammunition, with each shot impacting at a different location on the transparent armor system, without allowing penetration of any of the four shots through the transparent armor system. These new testing standards have resulted in the need to greatly increase the aerial mass and bulk of conventional transparent armor designs in order to allow such conventional transparent armor materials to meet the demands imposed by the new requirements and testing standards.
Moreover, several of the recently adopted requirements and testing standards relating to transparent armor use by the United States military require the use of transparent armor which is capable of withstanding subjection to ultraviolet radiation with minimal degradation of the transparent armor. Indeed, in several transparent armor applications, there is a need to use transparent armor in an environment in which the transparent armor is subjected to ultraviolet radiation, such as in vehicle windows and the exterior windows of a building. Ultraviolet radiation is harmful to conventional transparent adhesives used to bind a transparent hard face to the backup layer, with prolonged exposure resulting in degradation of the transparent adhesive. Conventional borosilicate float glass and soda lime glass structures are transparent to ultraviolet radiation, thereby allowing ultraviolet radiation to penetrate the hard face and contact the adhesive binding. The ultimate effect is that conventional transparent armor exhibits discoloration and delamination under prolonged exposure to ultraviolet radiation, such as in prolonged outdoor conditions.
There is a further requirement and need in several transparent armor applications to use transparent armor in an environment in which the transparent armor is subjected to extremes of temperature, for instance, in desert conditions. In some desert areas, nighttime temperatures can often fall well below freezing, and daytime temperatures inside an enclosed vehicle can exceed 85 degrees Centigrade (185 degrees Fahrenheit). In conventional transparent armor, differences in thermal expansion properties of the various layers of material can lead to delamination of the transparent armor when subjected to extreme changes in temperature.